The present invention relates generally to operating a fuel cell system, and more particularly to starting up and shutting down a fuel cell in such a way as to minimize oxidation of catalyst support material while maintaining system simplicity.
The use of catalysts to facilitate the electrochemical reaction between hydrogen and oxygen in fuel cells is well-known. Typically, the catalyst is in the form of a noble metal powder that is distributed on a support that is itself a powder of larger carbon or carbon-based particles. This powder-based approach allows for a significant increase in surface area upon which the aforementioned reaction can take place. While such a configuration provides for an efficient, compact reactor that by spreading the relatively expensive catalyst (such as platinum) over a large area results in significant improvements in power output with simultaneous reduction in raw material cost, its effectiveness can be limited by certain modes of operation. For example, even when the need for electric current produced in a fuel cell is reduced or ceases, the residual oxygen and hydrogen reactants continue to generate an open circuit voltage (typically around 0.9V or higher) that can lead to catalyst and catalyst support oxidation, thereby reducing fuel cell life. Of even greater concern is the presence of a hydrogen-air interface on one of the fuel cell electrodes (such as the anode) while air is present on the other electrode (such as the cathode), which can lead to potentials of between 1.4V and 1.8V being generated. These elevated potentials exacerbate the aforementioned corrosion of the catalyst and catalyst support material. This situation can occur during startup (when air is being purged by hydrogen) and during shutdown (when air is entrained into the anode as hydrogen is consumed by cross-over). The present inventors have observed that operational transients, particularly repeated system startup and shutdown, appear to shorten fuel cell life much faster than the comparable steady-state operation that takes place between such transients.
One way to alleviate the problem of residual fuel and oxidant is to inject an inert gas to purge both the anode flowpath and the cathode flowpath immediately upon cell shutdown. This could be accomplished by, for example, injecting onboard nitrogen into the anode and cathode flowpaths. However, this is disadvantageous, especially for many vehicle-based fuel cell systems, as the on-board use of a parasitic gaseous nitrogen supply would take up precious vehicle space otherwise used for passenger, comfort or safety features. Another approach is to introduce air into the anode flowpath so that the air can react with the residual hydrogen. By recirculating this mixture, the hydrogen can be ignited or catalytically reacted until virtually none remains. By this approach, no on-board nitrogen purge gas is required. However, this system is disadvantageous in that complex system componentry, including additional pumps coupled to intricate valve networks all tied together with precision control mechanisms, is required. Accordingly, there exists a need for a fuel cell system that can be started up and shut down without having to resort to approaches that require significant increases in weight, volume or complexity.